Production of low arsenic lime



Patented Feb. 14, 1939 i. iii 4 PRODUCTION OF LOW ARSENIG LIllIE Henry V. Moss, St. Louis, Mo., assignor to Monsanto Chemical Company, a corporation of Delaware No Drawing. Application August 14, 1937, Serial No. 159.098

4 Claims. (01. 23-186) This invention relates to the production of a ing in the usual way I pass the lump lime over pure form of quick lime, and particularly to a say a 1 inch mesh shaking screen and spray the form of quick lime having a low arsenic content. lime with a fine mist of water for say 1 minute.

I have found that in the ordinary method of I may interrupt the spray for one minute to perburning lime in vertical kilns where coal is used mit reaction of the water to take place. I repeat 5 as a fuel an appreciable amount of the arsenic the spraying with water at intervals say a total present in the fuel becomes associated with the of say five times and then permit the lumps to lime. I have also found that the arsenic thus stand for an additional period of 20 minutes. associated with the lime remains in the surface. At the end of this time the screen is shaken to layers of the lime lumps and that it may be subremove the loosely held partially hydrated outer stantially removed by partially hydrating and surface which has scaled off and which passes swelling the surface layers as by the addition of through the perforations in the screen. The cen- Water or steam. Such treatment of lump lime ters of the lumps on the screen are essentially results in the disintegration of the surface layer free of arsenic. In a typical experiment of this enabling said contaminated layer to be removed kind I have obtained the following arsenic values: and separated from the core of the lime without Arsenic in original quick lime, 1.4 P. P. M. effecting a substantial hydration of the latter. bone dry basis.

As an example of the extent of arsenic con- Arsenic in outer surface lime, 3.0 P.'P. M. bone tamination caused by a coal fuel, I have found dry basis.

that a typical hard burned lime will show an Arsenic in centers held on screen, 0.6 P. P. M. arsenic content upon the surface thereof at 4 bone dry basis.

P. P. M. as AS203 whereas the layer of lime /2 In this experiment, the outer surface which inch in from the outside surface has an arsenic was removed represents 27%of the original quick content of 1%,; P. P. M. which is typical 'of the lime, thus leaving 73% .of the lime essentially rest of the, lime in the core. I have found that arsenic free. The outer surface is partially hylime peeled 'ofi of the surface of lumps will in drated by the water to a lime having an ignition some cases run as high as 25 P. P. M. AS203. loss of9 [2% while the inner cores show an igni- In the practise of my process I have found that tion loss of only 4.8%. theremoval of the contaminated surface layers The high arsenic partially hydrated portion may be effected by abrasion as by tumbling the may be disposed of thru any suitable means 30 hard burned lumps of lime in a ball mill and where arsenic contamination or the need of removing the disintegrated powder which will moisture-free quick lime is not a factor, as for contain a large proportion of thearsenic present instance in the building industry or as agriculin the lime. As a preferred method, however, I tural lime.

may treat the hard burned lump lime with such The low arsenic lime obtained by my process a proportion of water that a hydration of the may be used in the manufacture of lime containsurface layers of lime is effected. Such hydraing food products and wherever a low arsenic tion results in a swelling and a formation of fairly content is necessary.

uniform scales of hydrated lime which scales The amount of lime removed from the outer readily falloff of the lumps of lime leaving besurface of the burned lumps will depend some- 40 hind a uniform hard burned core having a rela-' what upon the size of the lumps and upon the tively low ignition loss. arsenic content thereof. It is therefore subject As an example of one method of practising to some variation dependent upon these factors.

my invention, I give the following description: Having described my invention, what I claim Limestone is charged to a vertical ln'ln and is:

burned with an arsenic containing fuel such as 1. The process of decreasing the arsenic concoal until a hard-burned product is obtained. tent of lump lime which has been burned with The lumps of limestone are so chosen as to be arsenic containing fuels comprising disintegratof maximum size, consistent with the production ing the outer surface of said lumps and separatof a hard-burned product in the kiln employed ing the disintegration product from the remainand in any event should result in pieces of burned ing lumps,

lime which are considerably larger than 1 inch 2. The pr of d s n he rs ni conmesh. I prefer to obtain lime averaging in the tent of lump lime which has been burned with neighborhood of 4"-6 diameter pieces, arsenic containing fuels comprising disintegrat- Upon discharging the burned product and cool ing the outer surface of said lumP by the applicationof water thereto and separating the disintegration products from the remaining lumps.

3. The process of producing substantially arsenic-free lime comprising hard-burning lime-- stone lumps with arsenic containing fuels, cooling the hard burned lumps of lime and then disintegrating and removing the outer surface thereof. 7

4. The process of producing substantially arsenic-free lime comprising hard burning limestone lumps with a coal fuel and obtaining thereby hard-burned lime lumps having an outer shell of high arsenic lime and then removing said outer shell by disintegration.

HENRY V. MOSS. 

